1. Technical Field
The invention relates to lightguides, and is especially, but not exclusively, applicable to liquid and fiberbundle lightguides for transmitting ultraviolet light. The invention also relates to optical apparatus with such a lightguide coupled thereto, and to adapters for coupling them.
2. Background Art
Liquid and fiberbundle lightguides are typically used in applications where a large amount of radiation (typically UV, visible or IR) must be delivered to a remote location. Much of the art relating to the use of liquid lightguides for UV light transmission is directed to the curing of industrial photo-curable adhesives. This is because these applications require the application of intense ultraviolet or blue light to the chemicals being treated.
Liquid lightguides also have been employed with remote light sources for imaging applications, such as microscopy. In such an application, light from the guide usually is collimated or concentrated prior to illuminating a sample placed in the objective field conjugate of the microscope. In the case of a fluorescent microscope, the illumination of the sample by UV light allows for the visualization of molecules that are capable of fluorescing.
A typical liquid lightguide is composed of a core containing a highly transmissive liquid surrounded by a sheath that is generally made of a material having a low refractive index, such as Teflon™. Total internal reflection of the light occurs at the boundary between the liquid and the surrounding sheath. Transparent sealing members, such as glass rods, are placed at both ends of the guide to make a liquid-tight seal.
Many applications of lightguides require a very uniform irradiation distribution. This requirement is especially onerous where lightguides are used for imaging applications requiring a very uniform irradiation distribution across the field of view of the objectives. The present inventors have discovered that one reason why this has been difficult to achieve in practice is the presence of virtual images that are produced by the guide. In the case of liquid lightguides, the images arise from light scattering that occurs due to non-uniformities, such as any imperfections at the sheath, the sealing member, or at the interface between the liquid and the sealing member.
For example, at the interface between the liquid and the sealing member, air gaps are created by an uneven wetting of the liquid at the interface. The result is the creation of a number of mirror-like images along the transverse plane coincident with the sealing member. As this virtual source plane is imaged at a finite distance from the lenses, the outcome is a non-uniform irradiance distribution across the objectives of the microscope. Such images are well documented for sealing members, usually known as integrating or mixing rods, in the form of rods having straight edged cross-sections such as squares and hexagons, (see W. J. Cassarly, Handbook of Optics Vol. 3, Second Edition, Ch. 2, p. 2.28–2.33, McGraw-Hill, 2001, editor M. Bass), but the same effect applies to cylindrical rods, which are commonly used to seal the ends of tubular liquid lightguides.
Fiberbundle lightguides made of some type of glass or quartz material also are used for imaging applications, and they also exhibit undesirable non-uniformities which scatter light and can be imaged onto the image conjugate plane of the imaging system. In the specific case of fused fiberbundles, for which the individual fibers at the distal end of the lightguide have been mechanically fused together, the non-uniformities can be due, for example, to the contact region between the end of the fused bundle and a quartz rod, or to irregularities or inhomogeneity in or at the fused region of the bundle.
It is known to obtain a more uniform irradiance distribution by placing a diffuser at the end of a liquid lightguide. U.S. Pat. No. 5,684,908 discloses a system for photocuring of chemicals in which such a diffuser changes the shape of the light beam such that the majority of the beam is of a uniform intensity. A drawback to the use of such diffusers, however, is that the resulting optical coupling efficiency to the object to be illuminated, for example, a sample in a microscope, is less than optimal.